This invention relates to an improved bubbler system for rendering useful most of the liquid in a bubbler, and is particularly advantageous in connection with high purity liquid source materials used in the manufacturing of semiconductor devices.
In the manufacturing of semiconductor devices, a gas stream, saturated with an active compound is entered into a diffusion or chemical vapor deposition furnace. The active compound called a liquid source material contains an element which is diffused into or deposited upon a suitable substrate within the furnace. For example, boron tribromide can serve as a liquid source material for diffusion of boron into silicon or to deposit a borosilicate glass or boron itself, upon a suitable substrate. Typically, other liquid source materials used in semiconductor manufacturing include phosphorous oxychloride, phosphorous tribromide, silicon tetrabromide, arsenic trichloride, arsenic tribromide, and antimony pentachloride.
Since these materials readily react with air and to varying degrees, pose a health hazard to workers exposed to them, they require minimum or zero exposure to the atmosphere. Such nonexposure to the atmosphere is also required from a process control standpoint, since production yield is dependent upon the purity of the liquid source materials.
The function of a bubbler system in semiconductor manufacturing is to present a high-purity gas stream saturated witn an active compound to the diffusion or chemical vapor deposition furnace. It is highly desirable in relation to process control for the bubbler to deliver a constant amount of active compound per unit time to the using furnace. This constant delivery is accomplished by insuring complete saturation of the carrier gas with the liquid source material.
The saturation condition at a given temperature, may be defined as the state obtained when the partial pressure of the solute (the active compound) in a gaseous solution is equal to the vapor pressure of the pure solute at that given temperature. Therefore, saturation defines a maximum amount of solute which can be contained in the gaseous solution at equalibrium. Saturation is achieved by bubbling the carrier gas through the liquid source material to promote liquid-vapor interaction and transport of the liquid into the vapor. Obviously, saturation will be dependent upon many variables such as the gas bubble size, the path length for the gas bubbles through the liquid source material, and the residence time of the bubbles in the source material.
Presently, standard bubblers used in the semiconductor industry utilize a single chamber filled with liquid source material into which an inert carrier gas such as nitrogen or argon is circulated. For a given gas volume and bubble size, a certain minimum path length must be maintained in order to achieve complete saturation. Typically, this minimum path length is a distance of 1-11/2 inches of passage through the source material. Therefore, when the level of liquid source material falls below this minimum path length, saturation is no longer obtained and process control problems result.
Since various problems of contamination of the source material, discrepancies in source material lots, and health concerns are present in adding new source material to the bubbler, usual procedure is to discard the source material remaining in the bubbler and install a clean bubbler with a fresh charge, when the liquid level is below the minimum path length.
Due to this procedure of discarding the liquid source material after the minimum path length is lost, typically only 60 to 65 percent of the source material is utilized. Such source material waste, necessarily adds substantially to the cost of semiconductor devices.